Thoracic Imaging 2003 - Society of Thoracic Radiology

TUESDAY
186
Analysis of Mediastinal Contours
James C. Reed, M.D., F.A.C.R.
University of Louisville
Louisville, KY
Objectives:
1. Review normal mediastinal contours to improve perception
of mediastinal abnormalities.
2. Distinguish true mediastinal from paramediastinal abnormalities.
3. Review differential diagnosis of mediastinal widening and
masses based on alterations of mediastinal contours, location
and variations in opacity.
Perception of mediastinal abnormalities requires a thorough
knowledge of the many variations in the normal mediastinal contours.
There are numerous mediastinal abnormalities that may be
readily detected and diagnosed by plain film analysis; however,
many abnormalities are suspected from the radiograph and
require multi-planar imaging with CT or MR for confirmation.
Mediastinal contours are visualized as a result of the interface of
the mediastinal pleura with the lung. Abnormalities are perceived
when there are alterations in the normal mediastinal structures.
Additional structures such as an aneursym, masses, or cysts may
displace the pleura while invasive processes such as invasive cancers,
or lymphoma and some infections may cross the mediastinal
pleura eradicating the normal lung-pleura interface.
Analysis of mediastinal contours may be facilitated by dividing
the mediastinum into four quadrants and reviewing anatomy
on plain radiographs, CT and MR. The right superior mediastinal
contours are determined by the following: bracheocephalic
veins, superior vena cava, azygous vein, paratracheal line, and
the right main bronchus. The left superior mediastinal contours
include the broncheocephalic vein, subclavian vein and artery,
aortic arch, aorticopulmonary window, main pulmonary artery
and left pulmonary artery. The heart is the largest contour in the
inferior mediastinum, but the inferior vena cava and vertebrae
are identifiable on the right while the vertebrae, descending
aorta and paraspinal stripe should be visualized on the left.
Sagittal MR, the lateral plain film and axial images permit
examination with emphasis on anterior to posterior anatomy.
Anteriorly, we may identify abnormalities of the thymus, lymph
nodes, fat, and right heart. The middle structures include the
trachea, esophagus, superior vena cava, aorta, great vessels, and
lymph nodes. Posteriorly, we observe the descending aorta and
the spine.
Mediastinal lipomatosis is a common variant of normal seen
in obese patients. It is also seen in patients with endocrine disturbances
such as Cushing’s disease or those on high dose
steroid therapy. The opacity of fat is between that of the mediastinal
soft tissues and the aerated lung. This is sometimes recognized
on plain film, but usually requires CT for confirmation.
Since a number of mediastinal contours are the result of the
interface of vascular structures with the lung, there are important
differences in mediastinal contours based on the patient’s
age. This is especially true of the contours of the heart and great
vessels, especially the aorta and superior vena cava. Enlarged
vessels and vascular abnormalities must always be distinguished
from mediastinal masses especially before consideration of
biopsy.
Pulmonary consolidations should be distinguishable from
mediastinal abnormalities by the presence of ill-defined or irregular
borders and the presence of air-bronchograms or even
lucent spaces. Lung masses are often irregular and sometimes
heterogeneous, but may be smooth and when closely applied to
the mediastinal pleura could be difficult to distinguish from a
mediastinal mass. Lung masses which are firm and round may
form a characteristic sulcus with the mediastinal pleura This
appearance is often distinctive from the expected tapered interface
that is seen when a mediastinal mass displaces the pleura
into the lung. Primary lung cancers may be even more difficult
to evaluate because they are locally invasive and may extend
directly into the mediastinum or they may metastasize to the
mediastinal nodes. In contrast with invasive primary lung tumors,
mediastinal lymphomas arise in the mediastinal nodes or thymus
and may also be locally invasive and spread into the lung. Serial
films may document the progression of the tumor and provide
reliable signs for distinguishing lung tumors from lymphoma, but
this often requires biopsy when patients present with advanced
disease. Infrequently, other mediastinal tumors in particular
malignant thymoma may be locally invasive and mimic the
appearance of lymphoma or extensive metastatic disease.
Chest wall abnormalities may arise anteriorly from the sternum
or from posterior ribs and may suggest a mediastinal
abnormality on the plain film. They may be accurately identified
by the presence of bone destruction or by multiplanar
images that show extension of the abnormality into the chest
wall. Pleural abnormalities that arise from the medial pleura
may be more difficult to correctly localize and are often indistinguishable
from primary mediastinal abnormalities by plain
film and often require CT for correct diagnosis..
Masses are challenging to evaluate, but processes that
spread diffusely through the mediastinum produce an even less
distinctive radiographic appearance of diffuse mediastinal
widening. Mediastinal widening may result from hematoma,
vascular abnormalities, invasive masses, infection, fibrosis,
accumulations of fat and abnormalities of the esophagus.
While the expected appearance of adenopathy may be that of
circumscribed masses, very extensive adenopathy from both
infectious and neoplastic causes may diffusely widen the mediastinum.
Clinical correlation is essential, for example, patients
with AIDS who develop mediastinal adenopathy often have very
extensive adenopathy involving multiple node groups. This
requires consideration of infections by mycobacteria, or fungi,
and may also result from Kaposi=s sarcoma or lymphoma.
While reactive lymph node hyperplasia may cause progressive
generalized adenopathy this is not a frequent cause of nodes that
are detectable by plain film and rarely causes adenopathy with
nodes greater than 1cm on CT.
Careful analysis of mediastinal contours is required to distinguish
primary mediastinal from pleural, pulmonary and chest
wall abnormalities. Most mediastinal abnormalites are detected
with plain films, and in some cases the correct diagnosis may be
suspected, but most require CT, MR or biopsy.